BCS-BEC Crossover in Bilayers of Cold Fermionic Polar Molecules

TL;DR

This paper explores the phase diagram of fermionic polar molecules in bilayers, revealing how intralayer interactions influence the BCS-BEC crossover and superfluid properties at different densities.

Contribution

It combines weak and strong coupling theories to analyze the BCS-BEC crossover in bilayer polar molecules, highlighting the role of intralayer repulsion and many-body effects.

Findings

Intralayer repulsion broadens the BCS-BEC crossover regime.

Intralayer interactions can induce system collapse via roton softening.

The BKT transition temperature of the dimer superfluid is calculated.

Abstract

We investigate the quantum and thermal phase diagram of fermionic polar molecules loaded in a bilayer trapping potential with perpendicular dipole moment. We use both a BCS theory approach that is most realiable at weak-coupling and a strong-coupling approach that considers the two-body bound dimer states with one molecules in each layer as the relevant degree of freedom. The system ground state is a Bose-Einstein condensate (BEC) of dimer bound states in the low density limit and a paired superfluid (BCS) state in the high density limit. At zero temperature, the intralayer repulsion is found to broaden the regime of BCS-BEC crossover, and can potentially induce system collapse through the softening of roton excitations. The BCS theory and the strongly-coupled dimer picture yield similar predictions for the parameters of the crossover regime. The BKT transition temperature of the dimer superfluid is also calculated. The crossover can be driven by many-body effects and is strongly affected by the intralayer interaction which was ignored in previous studies.